In order to transmit a high-quality and high-capacity of data using limited radio resources in a wireless communication system, a variety of transmission methods have been developed and employed.
Closed-loop system provides a feedback channel from a user equipment to a base station. In the closed-loop system, when a user equipment provides channel information to a base station, the base station adjusts several system parameters, such as a power level and a transmission format, based on the channel information in order to maximize the performance. For example, an Adaptive Modulation and Coding (AMC) scheme is a method for increasing link performance by adjusting a modulation and coding scheme (MCS) based on the channel information. When channel state is good, the data rate is increased. On the contrary, when the channel state is not good, the data rate is lowered.
An example of channel information is a channel quality indicator (CQI). In general, the CQI is measured by a user equipment and feedback through an uplink control channel. The base station can assign appropriate resources to each user equipment based on the CQI.
For example, it is assumed that 5 bits (25=32 level) are used for CQI transmission.
CQI values between −10 dB and 22 dB can be quantized and represented every 1 dB distance. Or, CQI can be represented by one of 32 MCS levels. The 5 bits of CQI are channel-coded and then transmitted. If CQI is missed or distorted during transmission, AMC scheme can be much degraded. Accordingly, CQI needs to have high code rate to guarantee reliable transmission. But high code rate requires lots of radio resources.
In particular, in a multi-carrier system based on Orthogonal Frequency Division Multiplexing (OFDM), CQI may differ per on frequency (sub-carrier) basis. To acquire perfect channel information, it is needed that CQIs for every sub-carrier are measured and reported. But this may cause signaling overhead and low data rate.
Accordingly, there is a need for a method of efficiently transmitting CQI.